1. Field of the Invention
The present invention is directed to cellular materials. More specifically, the invention relates to syntactic foams comprising a polyurethane matrix resin and hollow polymeric microspheres, and the processes for making and using the same. The foams of the invention have utility as sound absorbing and decoupling agents.
2. Description of the Prior Art
The compositions of the invention are cellular materials that have been specially tailored to have sound absorbing properties suitable for muffling the sounds emitted by various underwater vehicles and surface ships and vehicles and to have acoustic scattering and decoupling properties so as to present an anechoic response when impacted by sound from an external source. More specifically, the compositions of the invention are syntactic foams prepared from a three-dimensional flexible or semi flexible polyurethane matrix and hollow polymeric microspheres having polystyrene shells.
Cellular materials, of which foams are an example, may generally be defined as two-phase gas-solid systems wherein the solid phase exists as a continuous matrix and the gas-phase occupies pockets dispersed throughout the matrix. The pockets, also known as cells or voids, may be discrete such that the gas phase within each cell is independent of that present in other cells. Cellular materials having discrete cells are denoted closed-cell foams. Alternatively, the cells may be partially or largely interconnected, in which case the system is termed an open-celled foam. Another type of classification of cellular materials is based on the bulk properties of the foam. A flexible foam, according to ASTM Test D 1566-82 (Definitions of Terms Related to Rubber, Vol. 37, ASTM, Philadelphia, Pa., 1982) is one that will not rupture when a 20.times.2.5.times.2.5 cm piece is wrapped around a 2.5 cm mandrel at a specified rate. In contrast, a rigid foam will rupture when subjected to the same conditions.
Yet another means to distinguish foams concerns the method of their manufacture. The most widely used methods involve dispersion of a gaseous phase throughout a fluid polymer phase and the preservation of the resultant foamed state as the gas expands while the fluid polymer precursor solidifies. In most foams, the expansion process consists of three steps. First is the creation of small gas bubbles in the fluid matrix. Next, these bubbles are expanded to a desired volume. Concurrently, the bubble filled fluid matrix is stabilized by conversion to a solid form through its polymerization.
In the case of thermoplastic foams, solidification of the melted matrix polymer results through cooling. Such foams are, however, not suitable for our application because they exhibit inadequate dimensional stability, do not possess the desired physical properties and cannot be cast in place. Cast in place directly onto a substrate, even of a highly irregular shape, provides essentially a perfect fit which cannot be achieved with a preshaped thermoplastic article. Furthermore, the highly undesirable use of adhesives is avoided using a cast in place system.
The creation of the small bubbles in the fluid matrix can be achieved through a variety of means. One common method of obtaining a blown foam is to incorporate a low boiling liquid into the fluid polymer precursor or prepolymer phase, or into fluid reactants and then vaporize the liquid blowing agent by increasing the temperature of the system or decreasing the ambient pressure. Concurrently, the fluid matrix solidifies entrapping the bubbles. An alternative method adds a chemical blowing agent to the fluid polymer precursor or prepolymer phase which decomposes and forms a gas upon heating. The concurrently solidifying matrix traps the gas bubbles. Another method which is popularly employed involves conversion of the fluid matrix to a solid form while a gaseous byproduct is being formed. Such is the case when polyisocyanates are reacted with polyhydric organic compounds and water to produce polyurethanes and carbon dioxide. In this case, the gaseous byproduct, carbon dioxide, is harnessed to produce the voids in the foam.
While polyurethane foams are commonly produced using byproduct carbon dioxide as the bubble forming agent, this technique and the other previously mentioned techniques are fraught with many practical difficulties when preparing a foam having the special cellular characteristics that we require. For example, the ambient conditions of temperature, pressure and humidity must be carefully monitored and controlled in order to control the rate and size of bubble formation. Other important parameters which must be considered include processing methodology such as the nature of the foam machine, type of mixing, mixing pressure, etc. and factors such as the smoothness of the surfaces to which the forming foam is contacted, and the surface characteristics of filler particles. Rough, irregular surfaces will tend to promote bubble formation. The presence of dissolved gases, such as air, in the polyurethane precursors will also contribute to bubble formation and so their potential contribution must be considered when developing reaction conditions. Unless the reaction and processing conditions are carefully monitored and controlled, the resulting foam may have undesirable cell volumes and/or cell volume distributions. One may also obtain open cells or a mixture of open and closed cells, as opposed to essentially all closed cells, which are required in our application. In the previously above described types of foams, the cell size distributions may be undesirable; however, in a syntactic foam the size of the added hollow particles may be controlled. Finally, in a blown foam, the cell shape may be elongated due to the anisotropic forces within the expansion process; whereas, the desired essentially spherical type symmetry is obtained for the hollow polystyrene spheres. The characteristics of the cells are very important because they significantly effect the structural, acoustic, thermal, and electrical properties of the foam.
In our application, we require closed-cell foams. Open-cell foams can fill with water when submerged for extended periods of time, especially at the high hydrostatic pressures that correspond to great depths. Except for syntactic foams, flexible polyurethane foams contain open or interconnected cells. Although rigid polyurethane foams are comprised primarily of closed cells, they cannot be used. Since they do not possess the desired acoustic characteristics, they undergo brittle fracture on slight to moderate impacts and undergo excessive irreversible compression sets or crushing when exposed to high compressive loadings.
The type of syntactic foams employed in this invention, use a flexible matrix polymer which possess the desired viscoelastic and other mechanical characteristics and also provides the desired isotropy.
One means to overcome some of the difficulties in controlling the size, shape and non-connectivity of the cells in a foam is to disperse microspheres in the fluid polymer precursor matrix. The product is known as a syntactic foam. The microspheres employed herein, are hollow spheres whose surface or wall structure has sufficient modulus so that the basic shape of the sphere will be retained in the finished foam. It is also known to make the sphere surfaces from phenolic resins, vinylidene chloride based polymers, urea-formaldehyde resin, glass or silica. It is also known to disperse these microspheres in epoxy resins, polyesters, and urea-formaldehyde based polymers. See Encyclopedia of Polymer Science, Cellular Materials, Volume 3, page 31, (John Wiley & Sons, Inc., 1985) .
In the preparation of the syntactic polyurethane one may employ either unexpanded microspheres, which are subsequently expanded during processing, or pre-expanded (already expanded) spheres. The former has the advantage of greater ease of incorporating the spheres into the matrix resin; whereas the latter provides for more precise control of the ultimate density of the polyurethane syntactic foam.
Instead of consisting of simple single-cell hollow (or gas-filled) spheres, as are those mentioned above, the individual polystyrene microspheres have inner hollow (gas-filled) cells which may number as many as 10,000 cells or voids per cubic millimeter. See reference K. Hinselman and Dr. J. Stabenow: "Zur Entstehung der Zellstruktur von Polystyrolschaum", Verein Deutscher Ingeniure, Berichte Nr. 182 pgs. 165-170, 1972. In a different type of application, J. D. Brooks and L. G. Rey: "Polystyrene-Urethane Composite Foam for Crash Padding Applications", pgs 233-235 in Journal of CELLULAR PLASTICS, September/October, 1973 involving crash padding, polystyrene beads are mixed into a polyurethane foam. Thus, in this reference, there are two types of voids present-those from the polyurethane foam and those from the expanded polystyrene beads. The semirigid composite foam that resulted had originated from a polyurethane foam that had to be flexible or at least semi-flexible. Such polyurethane foams are invariably open-celled which cannot be tolerated in underseas acoustic applications. Furthermore, the high set value of the composite foam on compressive loading (see FIG. 5 of the reference) also precludes its use in our type of application. In addition, the anisotropy of the cells of the polyurethane foam component also renders this type of material as unacceptable.